This invention relates to an ultrasonic pulse transmission method of detecting and mapping unbonded areas in a semiconductor device-to-structured copper bond.
In order to minimize electrical resistance heating and to maximize the conduction of heat away from the semiconducting layers, it is necessary that the interface of the semiconductor and heat sink be free from voids and unbonds. Since all bonding techniques such as soldering, brazing, diffusion, and thermal compression bonding can produce unattached areas and voids, it is necessary to detect the presence of such flaws and to determine their size, in order to make a proper quality assurance decision. The use of X-rays is not effective because the unbonded area usually does not result in a sufficient change in section thickness to be detected, even by computerized tomography. High frequency pulse-echo ultrasonic evaluations of phase inversions have proven to be an effective test of the semiconductor bond to tungsten and molybdenum heat sinks, but have failed to test devices using structured copper because of the high ultrasonic attenuation. An assembled device having a silicon wafer bonded both above and below to structured copper disks cannot, until now, be tested with high frequency ultrasound, X-ray, infrared, or electrical performance tests. The structured copper is pliable and formed of parallel, equal length, closely packed strands of copper, and accommodates to the difference in expansion between the semiconductor and fluid cooled metal heat sink.